In the conventional local loop telephone arrangement, each telephone in the loop is connected to the central office by a pair of copper wires. More specifically, a group of feeder wires is initially laid along a primary route, and telephones are connected to the feeder wires by distribution wires at various locations along the feeder wires, as required.
Local telephone companies expend a great deal of time and effort in deciding how much feeder wire to initially lay (i.e., how many wire pairs in the feeder), and over how long a distance. Projected population growth, creation of new housing and office developments, and new types of service which may be required in the future are some of the factors which are considered in this complex long-range planning determination. Since the cost of returning to install new wire at a later date may be greater than the cost initially installing more wire, the tendency is to initially install well beyond present requirements. However, copper wire is expensive, and wire which is ultimately unused represents economic waste.
The present invention provides a new local loop telephone system which to a large extent eliminates or minimizes the need for telephone company planning of the above-mentioned type or at least is so much more flexible that the need for precise planning is eliminated. Thus, the present invention provides a distributed, demand accessed system in the local loop wherein communication between the central office and remote units placed near subscriber populations is effected on an optical fiber. The combination of the optical communications mode and the demand accessing techniques of the invention result in a system of substantial communications capacity with the flexibility to adapt to dynamically shifting bandwidth requirements and easily lends itself to meeting long-term trends in the growth of capacity requirements.
The exciting possibilities for the use of optical fibers in communications systems has led to a wealth of prior art dealing with optical fiber communications systems. In the main, those systems have attempted to milk the last ounce of bandwidth from the optical fiber by using multiple optical carriers or space divisions techniques, decidedly different from that which is disclosed in this application. In this regard, see the following U.S. Patents:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,566,127 Hafner 3,633,034 Uchida et al 3,633,035 Uchida et al 3,851,167 Levine 3,883,217 Love et al 3,986,020 Kogelnik 3,995,155 Hutcheson et al 4,002,896 Davies et al 4,002,898 Milton 4,017,149 Kao 4,027,153 Kach 4,062,618 Steensma 4,089,584 Polczynski 4,145,109 Nelson 4,154,501 Fischer 4,161,650 Caouette et al 4,166,946 Chown et al 4,210,803 Ih 4,211,920 Wakabayashi 4,211,929 Tamburelli 4,215,269 Kuhn 4,223,216 Quick et al 4,225,753 Chown et al 4,227,075 Holland 4,227,260 Vojvodich et al 4,232,385 Hara et al 4,234,970 Beasley et al 4,236,243 Davies et al 4,237,550 Steensma 4,247,956 Christiansen et al 4,267,590 Bosotti 4,302,835 McMahon 4,326,298 Fromm et al ______________________________________
Furthermore, see "Lightwave Communications" by W. S. Boyle in the August 1977 issue of Scientific American, pp. 40 et seq; "Fiber Optic Communication - A Technology Coming of Age" by S. D. Personick appearing in the IEEE Communications Society Magazine, March 1978, pp. 12 et seq; "An Integrated Network Using Fiber Optics (INFO) for the Distribution of Video, Data and Telephony in Rural Areas" by Toms in IEEE Transactions on Communications, Vol. COM-26, No. 7, July 1978, pp. 1037 et seq; "Subscriber-Loop Digital Transmission Using Opto-Electronic Transmitters and Receivers" by Gorohov et al appearing in the IEEE Transactions on Communications, Vol. COM-27, No. 3, March 1979, pp. 629 et seq; "Digital Transmission Building Blocks" by S. D. Personick appearing in the IEEE Communications Magazine, January 1980, pp. 27 et seq; "Fiber Optic Transmission Link Analysis" by Campbell et al appearing in the Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 224, 1980, pp. 12 et seq; "Optical Channels in Distributed Processing" by Inbar in SPIE, Vol. 224 (1980), pp. 57 et seq; "A Rural Integrated Distribution Trial With Fiber Optics" by Kachulak appearing in the IEEE Communications Magazine, January 1981, pp. 36 et seq; "Toward Local Network Digitalization: The View from Japan" by Habara et al appearing in IEEE Transactions on Communications, Vol. COM-28, No. 7, July 1980, pp. 956 et seq. and "Loop Evolution - Its Dynamics and Driving Forces" by Homayoun in IEEE Transactions on Communications, Vol. COM-28, No. 7, July 1980, pp. 976 et seq.
Although there is a wealth of literature dealing with fiber optic communciations, the general application is to high capacity trunk routing. Personick concludes "[F]ibers are best suited for high density routes of sufficient length to justify the multiplexing cost and the high capacity."
At least one exception is Fromm et al. However, Fromm et al do not address multiplexing or concentration which are important features of the invention, as will be explained below.
In contrast to the prior art, the invention comprises a multi-access demand access distributed local loop in which in a preferred embodiment a fiber light media serves to connect an exchange with subscriber locations.
Using the invention, the telephone company can make an initial installation which is based on the total capacity which is ultimately expected rather than trying to plan with precision how much copper wire to lay for the foreseeable future. Since, with the invention, a small amount of fiber replaces a large amount of copper, in the conventional system, (fiber is much cheaper than copper), and since bandwidth can be dynamically reassigned, substantial amounts of excess capacity can be initially installed while still remaining economic. For example, with the system of the invention, a single fiber pair typically services over 1400 telephones which in a conventional local loop system would need to be serviced by 1400 separate wire pairs from the central office to the individual instruments.
The following example will provide an indication of the improvement afforded by the invention. If, for example, a requirement is seen to ultimately serve 1000 subscribers in an area served by a two mile-long feeder, the telephone company could, reasonably install one mile of 1000 pair feeder and 500 pair feeder for the second mile. If demand grows evenly, when fully loaded the feeder would be reasonably proportioned. However, it is simple to conceive of a situation wherein the entire demand developed in the first mile, leaving a 500 pair feeder entirely unused. Alternatively, if the entire demand was located in the second mile, a supplemental 500 pair feeder is required for the second mile. While these are simplified examples, the same problems are met in day-to-day telephone planning. With the present invention, a single two mile fiber satisfies future demand. Since bandwidth can by dynamically allocated, the single fiber can support 1000 subscribers in any distribution over the feeder without requiring mechanical changes, laying additional physical facilities or manual reconfiguration thereof.
Another disadvantage of the conventional local loop system is that the copper wire used is by itself a relatively low bandwidth medium, and the system can essentially only be used for telephone service, but not for higher bandwidth requirement service such as video. When such service is required, for example for teleconferencing or for the installation of a stock quotation system, it is necessary to prepare special facilities for each application.
In the future, it is believed that high bandwidth requirement applications such as teleconferencing will be in greater demand than in the past, and the installation of coaxial cable or other special facilities between the central office and the subscriber represents an important inconvenience and unwanted expense.
In accordance with a feature of the present invention, a subscriber can be assigned any (reasonable) desired bandwidth, so that video or other high bandwidth requirement service is possible without the necessity of additional installations or modifications. The optical communications medium used is inherently high bandwidth, and communication on the fiber is by a high speed data stream which is comprised of a number of communication channels, each of which is equivalent to a voice frequency (VF) channel. A subscriber requesting bandwidth may be assigned any required number of such channels up to the entire bandwidth of the high speed data stream, so that the subscriber's bandwidth needs are met.
The invention thus provides an optical, distributed, demand accessed local loop telephone system. In the prior art, it is believed that limited forms of bandwidth distribution techniques have been used in the local loop in connection with remote concentrators. See "Research Model for Time-Separation Integrated Communication" by H. E. Vaughan in Bell System Technical Journal, Vol. 38, No. 4, pp 909-932 (July 1959) and U.S. Pat. No. 4,230,913 for a description of a modern concentrator.
Typically, concentrators were used in a star configuration, i.e., plural remote concentrators were coupled to a central office over a dedicated concentrator link for each concentrator. Thus, it was necessary to predetermine the concentrator bandwidth limits which could not be exceeded without accomplishing physical changes in the concentrator or its dedicated link. In contrast, the remote access unit of the invention is coupled in a fiber loop or in series along a fiber feeder so that each remote access unit has access to the entire bandwidth of the fiber.
While remote concentrators are an improvement over the exclusive use of wire pairs they (1) do not entirely eliminate the necessity for accurate demand forecasting nor do they (2) eliminate the need for dedicated wire pairs between concentrator and subscriber. Accurate forecasting is still necessary to determine concentrator capacity and this capacity is typically fixed.
Employing a concentrator of relatively high capacity compared to initial demand in an attempt to reduce cost of expansion still represents economic waste due to the high cost of such concentrators. While the length of wire pairs from concentrator to subscriber can theoretically be reduced without limit, in practice the reduction of wire pair length is accompanied by multiplication of the number of concentrators. Again, because of concentrator cost, the length of wire pairs between concentrator and subscriber has been substantial.
In contrast, the low cost of the remote units of the present invention coupled with their ability to handle the entire fiber bandwidth (or capacity) is a substantial improvement over the use of concentrators.
A particularly significant advantage of the optical fiber transmission medium is the high bandwidth or data rate capability. For example a 16 Mbps pulse stream is used in a preferred embodiment. However, the same high rate means a reduced tolerance to jitter, i.e., the pulse width in the 16 Mbps stream is on the order of 60 nsec. This means that a jitter of .+-.30 nsec or greater can cause loss of data if the data must be referenced to an external clock such as may be required when synchronous processing is required. Furthermore, it is well known that jitter will accumulate at each of a plurality of nodes or retiming locations and thus the jitter is directly related to the length of the transmission link. This could be an intolerable situation absent some technique for rendering jitter manageable.